Various types of distributors which can be roughly classified into mechanical type distributors and suction type distributors are known. With the mechanical distributors, the surface of the which distributor is displaced continuously between the feed hopper and the transportation device is formed with recesses the volume of which corresponds substantially to that of the seed, the selector having as a function to expel or remove the seeds entrained by the distributor without being engaged into the recesses. With the suction distributors, the surface of the distributor providing the transportation is formed with orifices arranged in a row and connected to a negative pressure source, the selector expelling or removing the seeds which are not subjected to the suction effect of an orifice.
The disadvantages of the mechanical type distributors are that a specific distributor model is needed for each type of seed and even for each variety of the same plant, for example in the case of corn. In fact, the dimensions of the recesses have to correspond very precisely to those of the seed. This is even truer in the case of high precision mechanical distributors such as disclosed in U.S. Pat. No. 4,403,713 as although they bring about almost perfectly the absence of missing or duplicate seeds, without crushing any seeds, the dimensions of the various elements are narrowly determined by the size of the seed.
The disadvantages of the suction type distributors lie mainly in the fact that each orifice is likely to confine by suction several seeds at the same time, and that it is extremely difficult to remove the seeds in excess without having missing seeds. But a same diameter of the orifices permits distributing seeds having volumes varying within fairly large limits. In U.S. Pat. No. 3,796,346, a solution has been proposed to this problem, consisting, in a suction type distributor formed of a circular plate of horizontal axis in which the suction orifices open onto a face of the plate according to a concentric row, in providing two concentric rows of orifices, the orifices of the two rows situated on the same radius being close to each other, and a deflector arm sweeping the plate face at least accross the trajectory of the row of orifices which is most off-centre in order to move the seeds sucked by the orifices of said row and to pass them in front of the orifices of the second row so that, theoretically, a single seed be retained by suction by each orifice of said second row. The device is complex and, in practice, exclusively applicable to distibutors of horizontal axis, whereby a distributor can only provide the feeding of a single transportation device and therefore the setting in the soil of the seeds of a single row while the distributors of vertical axis the type considered in U.S. Pat. No. 4,403,713 can supply up to six and even to twelve transportation devices, and therefore a seed drill with six or twelve rows.